The present invention relates to an optical scanning device and an image forming apparatus using the same. More particularly, the invention relates to an optical scanning device of the type in which a light beam emitted from a light source is incident on a rotary polygon mirror while being slanted in the slow scan direction with respect to a plane that is oriented at a right angle to the rotary shaft of a rotary polygon mirror, and an image forming apparatus using the same.
The optical scanning device in use with the image forming apparatus, e.g., a laser printer, a digital copier or the like, is disclosed in the Unexamined Japanese Patent Application Publication Nos. Sho 64-79720 and Hei 7-27991. In the disclosed optical scanning device, a laser beam is incident on a mirror surface or facet of a rotary polygon mirror at a given incident angle with respect to a plane that is oriented at a right angle to the rotary shaft of a rotary polygon mirror. Then, the laser beam is reflected on the facet of the polygon mirror at a reflecting angle that is equal to the incident angle, while being deflected.
In the optical scanning device disclosed in the Unexamined Japanese Patent Application Publication No. Sho 64-79720, a light beam (or laser beam) emitted from a laser source is reflected by a reflecting mirror and incident on a rotary polygon mirror; the light beam from the polygon mirror is passed through a correction lens; the light beam emanating from the correction lens is successively reflected by a couple of reflecting mirrors; and the reflected light beam is projected on the surface of a photoreceptor drum. In the optical scanning device disclosed in the Unexamined Japanese Patent Application Publication No. Hei 7-27991, a light beam that is emitted from a light source is reflected by a plane mirror; the light beam successively passes through a couple of lenses; and the light beam emanating from the lenses is projected onto the facet of the polygon mirror. The light beam reflected by the polygon mirror passes through the two lenses again, and are incident on the surface to be scanned.
The optical scanning device disclosed in the Unexamined Japanese Patent Application Publication No. Hei 7-27991 has the following problem. With the beam width of the light beam when viewed in the slow-scan direction, the incident side mirror possibly interrupts an optical path of the reflection side mirror. To avoid the interruption, the incident side mirror must be disposed greatly apart from the polygon mirror. The result is that the optical scanning device is increased in size in the direction in which the light beam that is reflected by the polygon mirror advances.
In a case where an incident angle or a reflecting angle of the light beam with respect to the reflecting surface or facet 26A of the polygon mirror 26 is large, the light beam reflected and deflected by the polygon mirror 26 lands on the surface of the photoreceptor drum while depicting a locus along a curved surface. The result is that the light beam depicts a curved locus on the surface of the photoreceptor drum. (The behavior of the light beam under this condition is illustrated in FIG. 9.)
To avoid such the curving of the light beam, the necessity is to decrease the incident or reflecting angle of the light beam on the facet of the polygon mirror 26.
To decrease the incident or reflecting angle of the light beam on the polygon mirror facet 26A in the conventional optical scanning device including the first reflecting mirror for directing the light beam coming from the light source to the polygon mirror 26, and the second reflecting mirror for receiving the light beam from the polygon mirror 26 and directing it to the photoreceptor drum, those mirrors must be disposed such that the mirrors do not interrupt the related light beams or the optical paths of the same. Specifically, design necessity is to dispose those mirrors such that the first reflecting mirror does not interrupt the light beam reflected by the polygon mirror 26, and that the second reflecting mirror does not interrupt the light beam to be incident on the polygon mirror 26. In other words, the first and second reflecting mirrors are disposed so that the light beam reflected by the polygon mirror 26 is greatly spaced apart from the light beam incident on the polygon mirror 26.
Where the first and second reflecting mirrors are so disposed, the spatial distances between the mirrors and the polygon mirror 26 are greatly large. This entails an increase of the size of the optical scanning device.